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GNU Info (fdutils.info)XDFXDF formats =========== XDF is the format used for OS/2 installation floppies. Just like 2M, XDF uses mixed sector sizes on "generic tracks". The first cylinder uses 512 byte sectors. However, for XDF disks, the logical order of the sectors on a given track, and their physical order is not the same. This allows a faster access, in a similar way that interleaving does for disk with normal sized sectors and too small gaps. XDF's sector arrangement allows it to read sectors alternatively from both sides, i.e. the first sector from side 0, the second from side 1, and the third from side 0 again. This differs from the usualy formats, where first the entire side 0 is read, and then the entire side 1. This technique allows to read both sides of a disk in roughly three rotations. The following example illustrates how this is done. In our example we use the XDF format used for 3 1/2 HD disks, which contains one 8KB sector, one 2KB sector, one 1KB sector, and one 512 byte sector per track. The upper line represents the sectors on side 0, and the lower line represents the sectors on side 1. Different numbers represent different sectors. Repeated identical numbers represent a single larger sector. As the disk is circular, some sectors wrap around at the end: we find parts of the 8KB sector, represented by 6, both at the beginning and at the end of each track. position:| 1 2 5 4 | 1234567890123456789012345678901234567890 |========================================== side 0: | 6633332244444446666666666666666666666666 side 1: | 6666444444422333366666666666666666666666 2 512 byte sector 3 1KB sector 4 2KB sector 6 8KB sector When reading a track, sectors are read in the following order: sector id head position at start position at end ---------------------------------------------------------- 3 0 3 7 4 0 9 16 6 1 18 5 (1st wrap around) 2 0 7 9 2 1 12 14 6 0 16 3 (2nd wrap around) 4 1 5 12 3 1 14 18 We notice that the start of each sector happens at least 2 units of position (around 300 bytes), after the end of the previously read sector, thus allowing the floppy disk controller sufficient time to rest. Moreover, we notice two wrap-arounds, yielding three rotations to read the whole cylinder (the third rotation is due to the fact that we stop at a higher position than we started, and that we also need to allow some time for seeking to the next track). MSS or 2M formats of the same capacity nead at least 2 rotations per side (i.e. 4 per track), yielding a lower throughput. *Usage:* XDF disks are not bootable by LILO. They can be accessed from MS-DOS and OS/2 using `xdfcopy.exe' or `xdf.com'. They are only suitable for MS-DOS filesystems. The floppy driver has no direct support for this format yet, but `mtools' is able to read them using the `FDRAWCMD' ioctl. *Interesting Formats:* density tot. cap. throughput formatting command 5 1/4 HD 1600KB 46KB/s xdfcopy -0 /dev/fd0 3 1/2 HD 1840KB 38KB/s xdfcopy -1 /dev/fd0 3 1/2 ED 3840KB 102KB/s xdfcopy -2 /dev/fd0 The options `-1', `-2' and `-3' descibre one out of the five formats understoood by `xdfcopy' (3 XDF formats and 2 XDF formats). automatically generated by info2www version 1.2.2.9 |